The present invention relates to a continuous chromatographic process using a simulated moving bed structure of multiple zones. More specifically, the invention relates to focusing individual components of a mixture in different zones where said components are separated and concentrated.
Chromatographic separation can be classified into two modes, the batch-type mode and the continuous mode. Most of the conventional chromatographic applications are conducted by the batch-type mode. In general, the batch-type chromatography separates a sample by batches. The process for each batch comprises a sample loading step, an elution/fraction collection step, and a column regeneration step. The advantage of the batch-type separation is that it can separate complex mixtures. To insure a good resolution, sample loading should be less than 1% of the total capacity of a chromatographic column. The low sample loading is one of the disadvantages of the batch-type chromatography for production process. Since a new batch will not start until the whole process of the previous batch completes, batch-type chromatography also takes longer time for separating a defined quantity of products. Another disadvantage of the batch-type chromatography is the limited length of a chromatographic column that prevents from purifying closely related compounds.
Unlike the batch-type chromatography, the continuous chromatography is a process of continuous sample feeding, continuous elution, and continuous fraction collection. The sample feeding, the chromatographic elution, and the fraction collection are conducted simultaneously, which increases sample loading, enhances productivity, and saves eluent and chromatographic medium. A typical continuous chromatography is the simulated moving bed chromatographic system. FIG. 1 schematically shows a simulated moving bed chromatographic system. It comprises several zones that form a circular loop and are fluidly connected in series. The chromatographic columns are in these zones and can simultaneously relocate from one zone to the next in a unidirectional manner. There are two inlets and two outlets in different zones of the loop. Inlet F is for feeding a sample, inlet D is for introducing a desorbent, outlet R is for collecting a weakly absorbed component (raffinate), and outlet E is for collecting a strongly absorbed component (extract). A unidirectional fluid flow is maintained in the loop and is countercurrent to the direction of the column relocation. A sample is continuously fed into the system through the sampling inlet F, and the weakly absorbed component flows out of the system from the outlet R. The strongly absorbed component stays in the column and is moved with the column to the next zone upstream, where it is eluted out through the outlet E. The simulated moving bed chromatographic system has been successfully used for process purification of binary mixtures, such as enantiomer pairs, sucrose and non-sugar fraction, and xylene and its isomers. The main advantage of the simulated moving bed chromatography is the exceptional high throughput that makes the chromatographic separation economical. However, the simulated moving bed chromatography is limited to the separation of binary mixture and cannot be used to separate multiple components from complex mixtures. It even cannot be used for purifying only one component from a complex mixture if the said component is not the weakest absorbed or the strongest absorbed component in the mixture.
The limitation for the simulated moving bed chromatography is from its separation mechanism. A component of a mixture will experience two movements in the simulated moving bed system. The desorbent elutes the component downstream and the column relocations bring the component upstream. If the desorbent flows faster, the component will have an overall downstream movement. If the column relocations take place more frequently, the component will have an overall upstream movement. When two components are introduced from inlet F into the system, they will have different downstream movement speeds if they have different absorption strengths toward the columns. If the flow rate of the desorbent and the frequency of the column relocation are adjust to such a degree that the overall movement of the strongly absorbed component is in upstream direction and the overall movement of the weakly absorbed component is in downstream direction, then the weakly absorbed component will be collected from downstream outlet R and the strongly absorbed component will be collected from upstream outlet E. If a mixture contains more than two components, they will move either upstream to contaminate the strongly absorbed component or downstream to contaminate the weakly absorbed component. The attempt to have more zones and more outlets in the simulated moving bed system for separation of complex mixtures does not work either due to the same reason. The simulated moving bed chromatography also fails to purify a single component from a complex mixture if said single component is not the strongest or the weakest absorbed component in the mixture since said single component would be co-eluted with other impurities. Therefore, the separation mechanism has to be different from that of the simulated moving bed chromatography in order to separate a complex mixture.
The simulated moving bed chromatography is a dilution process and the concentration of the separated components in the collected fractions are dozens-fold lower than that in the sample. In order to obtain a separated component of high concentration and to save eluent, several prior arts describe a modified simulated moving bed chromatography with gradient elution. U.S. Pat. Nos. 4,031,155, 4,031,156, U.S. patent application Ser. No. 20020017492, and WO0033934 describe a desorption gradient that has two elution strengths along the fluid flow and is generated by the difference of desorption strengths between the desorbent and the sample. This modification is claimed to save the desorbent and to obtain a fraction with a higher concentration of the separated component than its concentration in the sample. However, the separation mechanism is the same as the classical simulated moving bed chromatography and is still a binary separation process. The function of the gradient is to make the upstream-moving component eluted out of the system faster and to use less desorbent. The same type of gradient is also described by Dorota Antos et al. (Chemical Engineering Science, vol. 56, (2001) 6667-6682), Thomas B. Jensen et al. (Journal of Chromatography A, 873 (2000) 149-162), Stefanie Abel et al. (Journal of Chromatography A 944 (2002) 23-29), Joukje Houwing et al. (Journal of Chromatography A, 944 (2002) 189-201), Dorota Antos et al. (Journal of Chromatography A, 944 (2002) 77-91), and Joukje Houwing et al. (Journal of Chromatography A, 952 (2002) 85-98). U.S. Pat. No. 6,069,289 describes another gradient process that has two desorbents and is claimed to separate a mixture into three fractions. It emphasizes on the fractioning of the mixture and dose not have sample concentrating effect since the process requires that the volume of the second extract is 2.2 to 10-times of the sample feeding volume, which means 2.2 to 10 times dilution of the sample after chromatography. The mechanism is again the same as that of the classical simulated moving bed chromatography.
A process for continuous chromatography that is based on a mechanism other than the mechanism for the classical simulated moving bed chromatography is needed for the separation of a complex mixture. The process should be able to purify complex mixtures, should be a continuous process of high productivity and low production cost, should significantly save the chromatographic media and the eluent, should have higher yield, and should obtain fractions in which the separated components have higher concentrations than in the feeding sample.
The key feature of the present invention is the formation of a focusing section (F section) for separation of complex mixtures by a new mechanism called simulated moving bed chromatographic focusing. The F section comprises f zones that are fluidly connected in series and are sequentially numbered from zone F(1) to zone F(f). A column for chromatography is detachably installed in each zone. A stream of main eluent enters zone F(f) and exits zone F(1). The main eluent is modified in such a way that it decreases its desorption strength as it leaves a zone and enters an adjacent downstream zone. It can be viewed that at least a few zones of the F section have eluents different from each other in terms of desorption strengths. The zone series can be viewed as a circle with zone F(1) adjacent to zone F(f) although zone F(1) and zone F(f) may not be fluidly connected. The columns periodically relocate from the current zones to the corresponding adjacent zones in a direction against the flow of the main eluent. The components of a sample mixture are introduced into a downstream zone, adsorbed on the column in the zone, brought zone by zone upstream, and desorbed from different zones one component after the other to get separation. A focused component is kinetically retained between zone F(i) wherein the eluent has lower desorption strength and zone F(i +1) where the eluent has higher desorption strength. The component in zone F(i+1) is strongly desorbed and eluted downstream by the main eluent to zone F(i) where it meets a new introduced mixture and reabsorbed in the column located in zone F(i). The component is thus enriched and is brought to zone F(i+1) again by column relocation. Due to the repeatedly column relocation and the continuous sampling, the component is gradually accumulated in between zone F(i) and zone F(i+1). If a small fraction of the main eluent is collected from in-between zone F(i) and zone F(i+1), a highly concentrated and purified component is obtained.
The present invention realizes separation of a complex mixture by a continuous chromatographic process, which has not been fulfilled practically by the existing chromatographic methods. The process of the present invention significantly enhances productivity, decreases production cost, increases product yield and purity, and reduces post-chromatographic treatment. Compared to the batch-type chromatography, it significantly saves chromatographic medium, solvent, and separation time and significantly increases product yield. Compared to the simulated moving bed chromatography, the present invention has the advantage of separation of complex mixtures and provided a separated component of higher concentration.